Problem: The Ponde family's Powerjet pumps 420 gallons of water per hour. At this rate, how many gallons of water will it pump in 45 minutes?
Solution: Since 45 minutes is $\frac{3}{4}$ of an hour, the Powerjet will pump $420\times\frac{3}{4}=\boxed{315}$ gallons of water in 45 minutes.